1. Technical Field
This invention relates to methods for non-invasively determining biological tissue oxygenation in general, and to non-invasive methods utilizing near-infrared spectroscopy (NIRS) techniques for determining the same in particular.
2. Background Information
U.S. Pat. Nos. 6,456,862 and 7,072,701, both assigned to the assignee of the present application and both hereby incorporated by reference, disclose methods for spectrophotometric blood oxygenation monitoring. Oxygen saturation within blood is defined as:
                                          O            2                    ⁢                                          ⁢          saturation          ⁢                                          ⁢          %                =                                            HbO              2                                      (                                                HbO                  2                                +                Hb                            )                                *          100          ⁢          %                                    (                  Eqn          .                                          ⁢          1                )            These methods, and others known within the prior art, utilize variants of the Beer-Lambert law to account for optical attenuation in tissue at a particular wavelength. Relative concentrations of oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb), and therefore oxygenation levels, within a tissue sample are determinable using changes in optical attenuation:
                              Δ          ⁢                                          ⁢                      A            λ                          =                              -                                          log                ⁡                                  (                                                            I                                              t                        ⁢                                                                                                  ⁢                        2                                                                                    I                                              t                        ⁢                                                                                                  ⁢                        1                                                                              )                                            λ                                =                                    α              λ                        *            Δ            ⁢                                                  ⁢            C            *            d            *                          B              λ                                                          (                  Eqn          .                                          ⁢          2                )            wherein “Aλ” represents the optical attenuation in tissue at a particular wavelength λ (units: optical density or OD); “I” represents the incident light intensity (units: W/cm2); “αλ” represents the wavelength dependent absorption coefficient of the chromophore (units: OD*cm−1*μM−1); “C” represents the concentration of chromophore (units: μM); “d” represents the light source to detector (optode) separation distance (units: cm); and “Bλ” represents the wavelength dependent light scattering differential pathlength factor (unitless)
To non-invasively determine oxygen saturation within tissue accurately, it is necessary to account for the optical properties (e.g., absorption coefficients or optical densities) of the tissue being interrogated. In some instances, the absorption coefficients or optical densities for the tissue components that create background light absorption and scattering can be assumed to be relatively constant over a selected wavelength range. The graph shown in FIG. 1, which includes tissue data plotted relative to a Y-axis of values representative of absorption coefficient values and an X-axis of wavelength values, illustrates such an instance. The aforesaid constant value assumption is reasonable in a test population where all of the subjects have approximately the same tissue optical properties; e.g., skin pigmentation, muscle and bone density, etc. A tissue interrogation method that relies upon such an assumption may be described as being wavelength independent within the selected wavelength range and subject independent. Our findings indicate that the same assumption is not reasonable, however, in a population of subjects having a wide spectrum of tissue optical properties (e.g., a range of significantly different skin pigmentations from very light to very dark) unless consideration for the wide spectrum of tissue optical properties is provided otherwise.
What is needed, therefore, is a method for non-invasively determining the level of oxygen saturation within biological tissue that accounts for optical influences from the specific tissue through which the light signal passes.